The basic premise of the Parallel Architecture (Jackendoff 1997, 2002) is that phonology, syntax, and semantics are independent generative components in language, each with its own primitives and principles of combination. The theory builds on insights about linguistic structure that emerged in the 1970s. First, phonology was demonstrated to have highly articulated structure that cannot be derived directly from syntax: structured units such as syllables and prosodic constituents do not correspond one-to-one with syntactic units. Moreover, phonological structure includes several independent substructures or tiers, each with its own type of generative structure: segmental-syllabic structure, the metrical grid, intonation contour, and (in tone languages) the tone tier. The tiers are correlated with each other by interface rules: principles that establish optimal correspondence between structures of two independent types. Such rules are not derivational. Since these phonological structures cannot be derived from syntactic structures, the connection between syntax and phonology must also be mediated not by derivations, but by a component of interface rules.